Cherry-pick merge r458643-1476567 from branches/cache-server to /trunk.
These patches minimize the membuffer cache addressing overhead and
implement a more sophisticated caching scheme. Both are essential for
fsfs-format7 performance.

Not intended for 1.8 back-port.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1476664 13f79535-47bb-0310-9956-ffa450edef68